Method and apparatus for exposure of flexographic printing plates using light emitting diode (led) radiation sources

ABSTRACT

A method and apparatus to expose photosensitive printing plates with a predetermined radiation density from the main side (top) and a predetermined radiation density from the back side (bottom). The method comprises executing the main exposure with a time delay after the back exposure. The time delay between back exposure and main exposure is optimized to create smaller stable single dot elements on the photosensitive printing plate after processing and smaller single element dot sizes printed on the print substrate. The plate floor may be adjusted by performing a back-side-only exposure prior to executing the combined back and main exposure with the time delay.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/926,616, titled PROCESS AND APPARATUS FOR CONTROLLED EXPOSURE OFFLEXOGRAPHIC PRINTING PLATES AND ADJUSTING THE FLOOR THEREOF, filed Mar.20, 2018, which is a continuation-in-part of PCT Application Ser. No.PCT/IB2016/001660, titled SYSTEM AND METHOD FOR CONTROLLED EXPOSURE OFFLEXOGRAPHIC PRINTING PLATES, filed 26 Oct. 2016, which claims priorityto U.S. Provisional Patent Application Ser. No. 62/246,276, filed on 26Oct. 2015. This application also claims priority to U.S. ProvisionalApplication Ser. No. 62/473,784, titled “PROCESS AND APPARATUS FORADJUSTING THE FLOOR OF A FLEXOGRAPHIC PRINTING PLATE IN A CONTROLLEDEXPOSURE SYSTEM OR PROCESS,” filed 20 Mar. 2017. All of the foregoingare incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Many processes are known in the art for preparing polymer printingplates, such as photopolymer flexographic plates and letterpressprinting plates coated with photopolymer material. One known processstarts with a plate having an ablatable material thereon, imaging theplate in a digital imager to ablate the ablatable material according toimaging data, and then curing the exposed plate by exposure of the plateto radiation, such as light energy, including but not limited toultraviolet (UV) light energy.

Various processes for curing the plate on both the imaged side and theback side of the plate by exposure to a functional energy source areknown, including methods for providing a blanket exposure (such as withfluorescent light tubes that emit UV light), and methods for providingthe desired radiation using light emitting diode (LED) technology, suchas is described in U.S. Pat. No. 8,389,203, assigned to the assignee ofthe present application and incorporated by reference. One particularlyuseful LED arrangement is shown and described in U.S. Pat. No.8,578,854, also incorporated herein by reference.

Known processes include exposing the back of a plate, then performinglaser ablation on the front side of the plate, then performing frontside exposure. Other processes include laser ablating the front side ofthe plate, then curing one side of the plate using a blanket exposure,manually flipping the plate, and then curing the other side of theplate. Each of the foregoing processes interposes an undefined, variabletime delay between the first and second exposure, depending upon theamount of time for the laser ablation step in the first process, ordepending upon the time it takes to manually flip the plate, in thesecond. This variability in elapsed time between first and secondexposure leads to undesirable variability in plate quality. Still otherprocesses may include exposing both the back side and the front side ofa plate simultaneously, which although it produces more predictableresults than a process that imposes a variable time delay, is still notoptimal, as discussed more herein later.

In the field of printing, minimizing the size of a dot printed on asubstrate is desirable, but smaller dots correspond to smaller printingplate elements, which are more susceptible to damage during use.Accordingly, there is always a need in the art to reduce the size orprinted dots, while also providing optimal stability of the printingelements on the plate for making those printed dots.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic drawing depicting an exemplary apparatus for theback exposure of a photosensitive printing plate in accordance withaspects of the invention.

FIG. 1B is a schematic drawing depicting an exemplary apparatus, havingtwo front-side heads and one back-side head, for the back exposure of aphotosensitive printing plate in accordance with aspects of theinvention.

FIG. 2A depicts a “single element number 64” as referred to herein,comprising 8 by 8 single pixels.

FIG. 2B depicts a “single element number 144” as referred to herein,comprising 12 by 12 single pixels.

FIG. 3A is a photograph of a 3D perspective side view of an exemplaryprinting plate element.

FIG. 3B is a photograph of a top view of the exemplary printing plateelement of FIG. 3A.

FIG. 3C is a photograph of a top view of a dot printed on a substrate bythe printing plate element of FIG. 3A.

FIG. 4 is a table showing and depicting with photographs resulting dotdiameters corresponding to 64-pixel and 144-pixel single elementstructures exposed at various time delays from 0 to 1500 seconds.Numbers in this table use a comma as the decimal mark symbol to separatethe integer portion from the fractional portion of non-whole numbers.

FIG. 5 is a graph corresponding to the results of FIG. 4, illustratingthe dot ground diameter for the smallest processed single dot element ona printing plate versus time delay between back exposure and mainexposure for an exemplary set of processing conditions.

FIG. 6 is a graph corresponding to the results of FIG. 4, illustratingthe smallest printed dot diameter versus time delay between backexposure and main exposure for an exemplary set of processingconditions.

FIG. 7 is a schematic drawing depicting an apparatus having acylindrical configuration for the front and back exposure of aphotosensitive printing plate in accordance with aspects of theinvention.

FIG. 8 is a schematic drawing depicting an apparatus featuring a planarradiation source for the front and back exposure of a photosensitiveprinting plate in accordance with aspects of the invention.

FIG. 9 is a schematic drawing depicting an apparatus featuring a singlelinear radiation source for the front and back and front exposure of aphotosensitive printing plate in accordance with aspects of theinvention.

FIG. 10 is a flowchart depicting an exemplary method of the invention.

FIG. 11 is a schematic illustration depicting a flatbed embodiment ofthe invention.

FIG. 12 is a schematic illustration of a light source embodimentcomprising a plurality of units having a plurality of rows of pointsources.

FIG. 13 is a schematic drawing depicting a portion of the plate duringexposure.

SUMMARY OF THE INVENTION

One aspect of the invention comprises an apparatus for preparing aprinting plate, the apparatus comprising a plurality of radiationsources configured to emit the actinic radiation, each of the radiationsources comprises a light emitting diode (LED), such as an LED sourcesconfigured to emit actinic ultraviolet (UV) radiation. The plurality ofradiation sources includes one or both of a set of front LED sourcespositioned to emit radiation toward the front side of the plate, withthe plate disposed in a stationary position, and a set of back LEDsources positioned to emit radiation toward the back side of the plate.The plurality of front LED sources together define a collectiveirradiation field covering an area at least coextensive with the laterallength and lateral width of the plate, and the plurality of back LEDsources together define a collective irradiation field covering an areaat least coextensive with the lateral length and lateral width of theplate. One or both of the sets of front and back LED sources may bestationary. A controller is connected to the plurality of radiationsources and configured to activate the plurality of radiation sources toemit the actinic radiation in a predetermined pattern that includes atime difference between activation of at least a portion of the set ofback LED sources and activation of at least a portion of the set offront LED sources. A holder may be configured to receive the printingplate in the stationary position to receive incident radiation from theplurality of radiation sources. The holder may comprise a material thattransmits the actinic radiation, such as a horizontal surface relativeto a ground on which the apparatus is disposed of a holder that definesa cylindrical surface. In another embodiment, the holder may beconfigured to hold the plate in a vertical orientation relative to aground on which the apparatus is disposed. The apparatus of may includeoptics, such as mirrors, for directing and/or confining the radiationemitted from each set of LED sources.

The predetermined pattern may include all of the set of front LEDsources emitting actinic radiation simultaneously, all of the set ofback LED sources emitting actinic radiation simultaneously, or acombination thereof. The predetermined pattern may include a timedifference between activation of at least a portion of the set of backLED sources and activation of at least a portion of the set of front LEDsources. The plurality of radiation sources may be individuallycontrollable or controllable in subsets smaller than an entirety of thecollective irradiation field corresponding to each set. Thepredetermined pattern may comprise a sequence that mimics relativemotion between the irradiation field and the plate. The predeterminedpattern may preclude simultaneously irradiating the front and the backof the plate by respective LEDs spatially aligned with one anotherrelative to a same cross-sectional coordinate of the plate. Thepredetermined pattern may include activation of multiple portions of theset of front LED sources and the set of back LED sources simultaneously,such as in a pattern that mimics multiple carriages. The predeterminedpattern may include activation of all of the plurality of back LEDsources simultaneously and then all of the plurality of front LEDsources simultaneously, with a predetermined time delay applied betweeneach front and back exposure. The predetermined pattern may includeproviding a plurality of exposure steps at an intensity that is lessthan a full amount of actinic radiation required to cure a correspondingside of the plate to a desired degree of curing.

The apparatus may be configured to deliver a first radiation intensityin one exposure step and a second, lesser exposure intensity in anotherexposure step. The first radiation intensity and the second, lesserexposure intensity may be delivered with the same set of LED sources.The set of front LED sources has a first nominal radiation intensity andthe set of back LED sources has a second nominal radiation intensity.The predetermined pattern includes activation of the back LED sources atthe second nominal radiation intensity in one exposure step, andactivation of the back LED sources at a radiation intensity less thanthe second nominal intensity another exposure step.

Each set of LED sources comprises a plurality of discrete arrays havinga plurality of individual LED point sources on each array, with theplurality of LED point sources arranged in a plurality of lines. Theplurality of LED point sources on each array may be controllabletogether, individually controllable, or controllable in groups, such aseach line of LED point sources in each array being separatelycontrollable. The radiation intensity generated by at least one line ofLED point sources may differ from a corresponding radiation intensitygenerated by at least one other line of LED point sources for a sameamount of input energy, wherein the controller is configured to controlthe radiation intensity produced by each line to achieve an intendeddegree of homogeneity.

Another aspect of the invention comprises a method for calibrating theapparatus as described herein by periodically characterizing radiationintensity generated by a plurality of lines of LED point sources andadjusting input energy to the respective plurality of lines of LED pointsources to account for variations in the lines over time. Thecharacterization may be performed by positioning a sensor that measuresincident radiation at a predetermined distance from each line of LEDsources. The method may include tailoring radiation intensity of one ormore groups of the LED point sources to compensate for variations intransmissivity of a structure that lies between the sources and theprinting plate.

Still another aspect of the invention comprises a method for exposing aphotopolymer printing plate to actinic radiation from a UV LED lightsource, the photopolymer printing plate having a non-printing back sideand a printing front side with a mask for defining an image to beprinted. The method comprises the steps of (a) positioning thephotopolymer printing plate in an exposure unit, wherein the exposureunit comprises a plurality of UV LED radiation sources; and (b) exposingthe photopolymer printing plate through the mask to actinic radiationfrom the plurality of UV LED light sources to cure a portion of thephotopolymer in the plate in an exposure step during which the pluralityof UV LED radiation sources and the photocurable printing plate do notmove relative to each other. A first exposure intensity is provided inone exposure step and a second, different exposure intensity is providedin another exposure step. Where the exposure unit has optics, includingmirrors, the exposure step may comprise directing the radiation emittedfrom the plurality of UV LED radiation sources with the mirrors. Theplurality of UV LED radiation sources may comprise one or more sets ofLED sources, each set of LED sources together defining a collectiveirradiation field covering an area at least coextensive with a laterallength and lateral width of the plate. The plurality of UV LED radiationsources emit the actinic radiation at a first intensity, wherein theactinic radiation reaching a photocurable surface of the printing platemay be less than the first intensity, such as wherein a portion of theactinic radiation emitted by the plurality of UV LED radiation sourcesis not transmitted by a surface on which the plate is mounted, blockedby a substrate on which the photocurable polymer is disposed, or acombination thereof.

The first exposure intensity may be provided in one or more backexposure steps and the second exposure intensity may be provided in oneor more front exposure steps. The method may include performing thefirst exposure step with the plurality of UV LED light sources emittingat a nominal intensity, and the second exposure step with the pluralityof UV LED light sources emitting at less than the nominal intensity. Thefirst exposure step and the second exposure step may both be backexposure steps. The method may comprise providing a plurality of backexposure steps including at least a first back-only exposure step and asecond back exposure step followed by a predetermined time delayfollowed by a front exposure step, including adjusting the intensity ofthe one or more of the plurality of UV LED light sources in at least oneof the back exposure steps to provide a predetermined ratio Rbf betweenfront side and back side exposure of the printing plate.

DETAILED DESCRIPTION OF THE INVENTION

Those of skill in the art understand that oxygen is distributedthroughout the photopolymer resin of a polymer plate at the time it istypically processed, and that oxygen is an inhibitor of thepolymerization reaction commonly harnessed for curing the plates.Although polymerization caused by exposure of the polymer to actinicradiation scavenges this distributed oxygen, ambient oxygen will diffuseback into the resin over time if the plate is in contact withatmospheric air. Surprisingly, in processes in which a back exposure andmain exposure are both performed on a plate, it has been found that finedetail on a plate may be optimized by imparting a defined delay betweenperforming the back exposure and the main exposure. Without being heldto any particular mechanism, it is believed that in this defined delaytime following the back exposure, which scavenges oxygen from the backportion of the plate, oxygen from the front side of the plate starts todiffuse to the back side, thus creating a slightly less oxygen richconcentration in the area of the plate nearest the floor of the plate,such that the polymerization reactions near the floor of the plate reactfor longer before stopping and therefore create shapes on the plate thattaper from the floor toward the top of the plate following the mainexposure. It should be noted that a delay that is too long will resetthe entire plate to being oxygen saturated, and a delay that is tooshort may not permit sufficient oxygen diffusion to produce optimalresults. Thus, while the amount of the optimal delay may vary dependingon any number of characteristics, what is important is that the delaynot be too long or too short, for optimal results. This delay may beimparted in any number of ways, described in more detail herein.

An exemplary apparatus 100 for the back exposure of photosensitiveprinting plate 130 is shown schematically in FIG. 1. As is well known inthe art, printing plate 130 comprises a photosensitive polymer 134 onwhich is disposed a mask 132 that defines portions of the plate that aremasked from radiation exposure relative to portions of the plate thatare desired to receive such exposure. In a typical embodiment, thepolymer 134, including in the mask 132 area, is permeable to oxygen.

In apparatus 100, a UV source of actinic radiation 120 with apredetermined power density is scanned at a specific speed (v) under thebottom of the plate. For the main or front exposure of thephotosensitive printing plate a second UV source of radiation 110 with apredetermined power density (irradiance) is scanned above the plate withthe same specific speed (v). UV sources of radiation 110 and 120 areconfigured to scan the printing plate with the same speed (v). Such aconfiguration may be provided by synchronizing sources 110 and 120 tohave a same speed using a controller, or both sources may be attached toa common carriage that traverses the plate, with sources 110 and 120spaced apart from one another a suitable distance in the direction ofcarriage travel to provide the desired delay when the carriage moves ata predetermined speed. The predetermined irradiance may be the same forthe main side and the back side, or may be different. Preferably theirradiance at the rear side is only a fraction of the irradiance of thefront side exposure. Typically, the irradiance at the rear side is in arange of 10% or less of the front side irradiance, but the invention isnot limited to any particular ratio of front to back irradiance. Thepredetermined irradiance is typically a function of the characteristicsof the specific type of plate to be exposed, as is known to those ofskill in the art, and as is dictated by the manufacturers of suchplates.

The time delay between the back exposure with UV source of radiation 120and the main exposure with UV source of radiation 110 may be adjusted bythe control system 140 by adjusting the speed of the sources and/ormechanically by setting a constant distance (D) between the sourcesduring the scan process. The time delay t=D/v. Thus, mechanicallyvarying D has an impact on the delay, as does the relative speed betweenthe plate and the sources during exposure. The time delay can beoptimized to get smaller single dot elements on the photosensitiveprinting plate after processing and smaller single element dot sizesprinted on the print substrate. It should be understood that thearrangement depicted in FIG. 1 is schematic in nature only, to show therelationship between the light sources and the distance D relative to aplate. In a system 100 in which printing plate 130 is disposed along ahorizontal plane (i.e. in which directional arrow Y of the X-Y axisshown in FIG. 1 represents the directional pull of gravity), plate 130may be mounted on a transparent substrate 160 (such as glass). In asystem 100 in which printing plate 130 is disposed along a verticalplane (i.e. a system in which directional arrow X of the X-Y axisrepresents the directional pull of gravity), the plate may be hungvertically (such that no substrate under the plate or other structurebetween the radiation source and the plate are required), such as from ahanger 170. It should be understood that hangar 170 as depicted in FIG.1 is intended only to be schematic, and is not intended to represent anyparticular hangar geometry. Furthermore, although shown in a flatorientation, it should be understood that the printing plate may beflexible enough to be disposed around a transparent cylinder, such as aglass cylinder, or the plate may be in the form of a continuous sleeve,as is known in the art, with the distances between the light sourcesarranged relative to the rotational direction of the cylinder, asgenerally depicted in FIG. 7 and described in more detail herein later.

The relative movement between the radiation sources and the plate may beprovided by any mechanism known in the art for moving objects relativeto a horizontal, vertical, or otherwise disposed stationary surface. Forconfigurations in which the radiation sources move and the plate isstationary, for example, the sources may be disposed on a gantry systemhaving arms that pass the respective sources above and beneath astationary horizontal plate mounted on a substrate configured to permita sufficient amount of radiation to pass through, or on either side of avertically mounted plate. For configurations in which the radiationsources are stationary and the plate is movable, for example, the platemay be mounted on any mechanism known in the art, such as a movablestage configured to move relative to fixed sources on opposite sides ofthe stage. Mechanisms for rotating a cylinder on which a plate ismounted relative to fixed sources are well known in the field ofprinting. Similarly, mechanisms for rotating sources relative to a fixedcylinder on which a stationary object is mounted are also well known,such as in the field of medicine (e.g. CAT scan machines). Thus,mechanisms for moving one or more elements relative to another are wellknown in the art, generally, and the invention is not limited to anyparticular mechanism.

As shown in FIG. 1, it should be understood that each of the frontsource 110 the back source 120 have an irradiation field covering anarea at least coextensive with a width of the plate (wherein the “width”lies along the third dimension not shown in the 2-dimensional image ofFIG. 1) but not coextensive with a full length of the plate (wherein the“length” lies along the X-axis as shown in FIG. 1). Each of the frontsource and the back source may thus comprise a linear source (such assources 1120 and 1122 shown in FIG. 11) that emits radiation along aline parallel to the width of the plate. Each linear source, however,may comprise a plurality of subsources (such as LED point sources 1212shown in FIG. 12) that together collectively create the linear radiationfield having a defined length less than the length of the printingplate, and a width that spans at least the entire width of the printingplate.

In one embodiment, shown in FIG. 11, carriage 1130 may comprise a firstlinear source 1122 arranged to irradiate the back side of a plate 1114mounted on transparent surface 1112, such as a glass plate, and a secondlinear source 1120 arranged to irradiate the top side of the plate. Eachlinear source extends to cover one dimension of the plate, which in theexample shown shall be referred to as the transverse direction. Thecarriage traverses the plate in the longitudinal (or lateral) directionalong arrow L, with at least one source, and preferably both sources,activated. While the exposure step may be performed in a single pass, insome embodiments the exposure may be performed in a plurality of passes,in which each pass imparts radiation using both banks of sources at afraction of the total exposure needed to provide a desired amount ofexposure. As will be understood, the carriage may have a first speedwhen traversing the plate along the direction of arrow L with radiationsources activated, and a second, faster speed when traversing the platein the direction opposite arrow L, to reset for another pass or at thecompletion of the desired number of passes.

The overall mechanism for creating the exposure may comprise a tablehaving an outer frame 1110 that holds a transparent (e.g. glass) innerportion 1112. The upper 1120 and lower 1122 linear radiation sources(e.g. banks of LED point sources, optionally mounted inside a reflectivehousing) are mounted on a gantry system or carriage 1130. The radiationsources are connected to a power source, such as an electrical powercord having sufficient slack to extend the full range of motion of thecarriage. Tracks (not shown) disposed on the outer frame portion providea defined path for the gantry system or carriage to traverse. Thecarriage may be moved on the tracks by any drive mechanism known in theart (also coupled to the power supply and the controller), including achain drive, a spindle drive, gear drive, or the like. The drivemechanism for the carriage may comprise one or more components mountedwithin the carriage, one or more components fixed to the table, or acombination thereof. A position sensor (not shown) is preferably coupledto the carriage to provide feedback to the controller regarding theprecise location of the carriage at any given time. The control signaloutput from the controller for operating the radiation sources and forcontrolling motion of the carriage may be supplied via a wired orwireless connection. The controller may be mounted in a fixed location,such as connected to the table with a control signal cable attached tothe sources similar to the power cable, or may be mounted in or on thecarriage. The control system and drive mechanism cooperate to causeback/forth relative motion in a transverse direction between the lightfrom the radiation sources and the plate. If should be understood thatother embodiments may be devised in which the drive mechanism isconfigured to move the portion of the table containing the plate paststationary upper and lower linear radiation sources, as well asembodiments in which the radiation sources cover less than the fullwidth of the plate and are movable in both the transverse andlongitudinal direction to provide total plate coverage (or the plate ismovable in both directions, or the plate is movable in one of the twodirections and the sources are movable in the other direction toprovides the full range of motion required to cover the entire plate).

In one work flow configuration, the table for conducting the exposurestep (i.e. exposure table) as described above may be positioned toautomatically receive an imaged plate from an imager. For example, animager may be positioned so that the imaged plate expelled therefromlands in a first location, and a robotic handling device may beconfigured to automatically pick up and move the imaged plate from thefirst location to a processing location on the exposure table, where theexposure process as described herein is then performed using transverselinear sources attached to a carriage that traverses the platelongitudinally.

As discussed in U.S. Pat. No. 8,578,854 and illustrated schematically inFIG. 12, each bank 1200 of LED sources may comprise a plurality ofdiscrete units 1210 having a plurality of individual LED point sources1212 on each unit, with the plurality of point sources arranged in aplurality of lines 1220, 1222, 1224, 1226, 1228, 1230. All of the pointsources on each unit may be controlled together, may be individuallycontrolled, or may be controlled in groups. For example, each line ofpoint sources in each unit (e.g. each of lines 1220, 1222, 1224, 1226,1228, 1230) may be separately controllable. Providing such a fine levelof control may have several advantages. For example, the actual outputfrom each line of LEDs may vary slightly for the same amount of inputenergy, due to variations in the LEDs themselves, soldering to thecircuit board, cooling, decay or wear over time, and the like, and thus,each line of LEDs may be characterized and their intensity varied by anappropriate factor relative to other lines to so that the radiationoutput produced by each line is as close to homogenous as possible.Characterizations and re-calibration may be performed on a periodicbasis to account for variations in the lines over time. Suchcharacterizations may be performed by positioning a sensor that measuresincident radiation at a predetermined distance from each line of LEDsources. On top of compensation for variations in the output intensityof the LEDs themselves, further compensations may be made for variationsin transmissivity of any structure that lies between the sources and theprinting plate, such as for example, the glass surface 1112 that liesbetween the back sources and the printing plate in the configurationshown in FIG. 11. Any characterizable variations in transmissivity ofemitted radiation through the glass surface can be countered by varyingthe intensity of the LEDs based upon carriage location so that theamount of radiation that reaches the back of the plate is as close tohomogenous as possible over the entire exposed plate area.

Definitions

The term “single element structure number” as used herein refers to asquare defined by the total number of pixels that comprises that square.For example, a “64-pixel single element structure” comprises square 200,which comprises an 8×8 grid of pixels 202, and has a total of 64 pixels,as illustrated in FIG. 2A. Likewise “144-pixel single element structure”250 comprises a grid of 12×12 pixels 202, yielding a total of 144pixels, as illustrated in FIG. 2B.

The term “dot top diameter” refers to the diameter of the top of aprinting plate element or “dot” (i.e. the portion of the element thatcontacts the printing surface), as illustrated in FIG. 3A, showing aphotograph of a 3-dimensional perspective side view of an exemplaryprinting plate element 300 and its dot top diameter 310. The term “dotground diameter” refers to the diameter at the base of a printing plateelement or “dot” (i.e. the diameter of the element at the floor or“ground” of the plate), as illustrated in FIG. 3B, which is a photographof a top view of exemplary printing plate element 300 and its dot grounddiameter 320. The term “printed dot diameter” refers to the diameter ofthe dot that is printed on a substrate by a printing element, asillustrated in FIG. 3C, which is a photograph of a top view of printeddot 350 and its printed dot diameter 330.

Example

For optimization of the time delay, single element structures of varioussizes were imaged by a laser into the mask of a photosensitive printingplate at a resolution of 4000 dpi. For this example, a Model No. DPR 045printing plate, manufactured by DuPont, was used.

The photosensitive printing plates were then back exposed, such as byusing UV radiation source 120, and main exposed, such as by using UVradiation source 110, as depicted in FIG. 1. For this example, eachsource 120 and 110 source comprised a linear source comprising a bank ofindividual LED UV point sources, as described in more detail herein. Theplate was exposed in a single exposure step using a main side UVirradiance of 230 mw/cm² at a wavelength of 360 nm and a back side UVirradiance of 17 mw/cm² at the same wavelength at a relative plate speedof 1.25 mm/sec. For this example, the UV radiation sources were movedlengthwise under and above the surface of each photosensitive printingplate at the specified speed. The time delay was varied to optimize thesmallest single dot element on the processed photosensitive printingplate and printed to optimize the smallest printable dot size on theprinting substrate.

Results of exemplary time delays for exemplary single element structurenumbers 64 and 144 are shown in FIGS. 4-6. As shown in FIG. 5, a plot ofthe ground diameter of the smallest processed single dot element versusthe time delay between back exposure and main exposure for any set ofconditions yields a maximum 500 (i.e. 573.33 μm diameter at 92 secondstime delay, for the plot shown). Thus, the size of the base of the dot,and therefore the stability of the shape, can be optimized by optimizingthe time delay between back exposure and main exposure. As shown in FIG.6, a plot of the smallest printed dot on the substrate versus the timedelay between back exposure and main exposure yields a minimum 600 (29μm diameter at approximately 92 seconds time delay, for the plot shown).In general, the smallest printed dot size is desirable for highestresolution. In general, the smallest printed dot size with the largestdot ground diameter is optimal.

The optimized results shown in FIGS. 4-6 above are specific to theparticular printing plate system and other variables, such as speed,energy density, etc., for the example discussed herein. It should beunderstood to those of skill in the art that different printing platesystems, different speeds, different energy densities, and othervariables may impact the optimum results achievable by the processdescribed herein, and that similar graphs and optimums can be generatedfor any type of print system. In general, however, the delay timebetween the rear side exposure and the front side exposure may generallyfall in the range between 10 and 200 seconds, more preferably a rangebetween 2 and 100 seconds, and most preferably in a range of between 5and 20 seconds. Minimizing the delay time minimizes overall processingtime, and thus has an impact on overall throughput of a system.Accordingly, optimizing other conditions to minimize the time delay mayalso be beneficial.

Although the exemplary system shown in FIG. 1 illustrates the time delayschematically in a linear system, it should be understood that variousexposure systems may be devised to provide the optimized time delay. Insuch exemplary systems, the UV light sources may comprise, for exampleand without limitation, LEDs, arrays of LEDs, fluorescent lights, suchas fluorescent tubes, arc discharge lamps, or any other UV light sourceknown in the art. Although described herein in connection with a UVsystem and referring to “UV light”, it should be understood that thetechnology described herein is not specific to any particular type ofradiation wavelength, visible or non-visible, and that the system mayutilize any type of actinic radiation or other radiation that isfunctional to cause the photochemical reaction necessary to cure thetype of plate used. Thus, the term “light source” as used herein refersto any type of actinic radiation source.

In one embodiment 700 depicted in FIG. 7, the printing plate 730 may bemounted on a transparent (e.g. glass) cylinder 760 rotating at apredetermined speed, with the main radiation source 710 disposed in afirst location along the cylindrical path of rotation adjacent theexternal surface of the cylinder, and the back side radiation source 720disposed in a second location along the cylindrical path of rotationadjacent the internal surface of the cylinder, with the respectivelocations of the sources spaced apart by the distance required toprovide the time delay required at the speed of rotation. In such asystem, the location of the light sources and/or the speed of rotationmay be variable to provide different time delays. The photosensitiveprinting plate 730 may be a sleeve, such as a sleeve designed to fitover the transparent cylinder 760 of the system described above, or maybe flat, but sufficiently flexible, to permit it to be disposed on andsecured to the surface of the cylinder. It should be understood that theterm “transparent” as used herein may refer to any material that permitsa desired amount of radiation at the desired wavelength pass through theselected material. Thus, “transparent” as used herein, may refer to amaterial that is not visibly transparent or even translucent to thehuman eye.

In another exemplary embodiment 800, depicted in FIG. 8, each collectiveradiation source 810, 820 may emit a planar radiation field that is atleast coextensive with both lateral dimensions of plate 830 (e.g. eachcollective radiation source 810, 820 may be configured to irradiate theentire plate surface all at once when activated, if configured to beactivated in that manner), in which case the controller 850 may beconfigured to create a delay time by creating a time difference betweenturning on a portion of source 820 for exposing the back surface andturning on a portion of source 810 for exposing the main surface. Theprinting plate 830 may lay flat on a horizontal transparent (e.g. glass)plate 860 or may hang in a vertical orientation, such as from a hangar170 as depicted in FIG. 1. Although depicted schematically as singlecontinuous sources 810, 820 in FIG. 8, each source 810, 820 preferablycomprises a plurality of individual subsources (not shown), such asfluorescent tubes or LED point sources that are individuallycontrollable or controllable in subsets smaller than the overallirradiation field. The plurality of subsources may be coordinated andcontrolled to act as a single source, or individually activated in adesired pattern. For example, in a configuration comprising a pluralityof stationary subsources and a stationary plate, the individualsubsources may be independently controlled so that fewer than all of theindividual subsources comprising source 810 are turned on at the sametime and fewer than all of the individual subsources comprising source820 are turned on the same time. The collective subsources may thus becontrolled in any pattern that provides the desired time delay andavoids simultaneously irradiating the front and the back of the plate bysubsources that are spatially aligned with one another relative to thesame coordinates of the plate.

One exemplary control pattern may activate the radiation subsources in asequence that causes relative motion between the radiation field and theplate, such as a movement that essentially mimics the same lightpatterns that would be provided by main and back linear sources attachedto a carriage, but with the advantage of having no moving parts. Theillumination pattern may be configured to illuminate multiple portionsof the front and back simultaneously (e.g. such as in a pattern thatmimics multiple carriages—one starting at one end of the plate, and onestarting in the middle). The illumination pattern in such aconfiguration is not constrained to patterns that mimic one or morecarriages, however, and may be implemented in any pattern that providesthe desired time delay, overall exposure, and lack of simultaneousexposure from front and back for any particular cross sectionalcoordinate of the plate. The pattern may also comprise illuminating theentire back at once and then the entire front, either in a singleexposure for each side, or in fractional exposures of the full requiredexposure for each side, with the desired time delay applied between eachfront and back exposure. Furthermore, although shown in a flatconfiguration, it should be understood that systems in which both theplate and the sources are stationary may also be arranged in acylindrical configuration.

Optionally, the embodiment shown in FIG. 8 may also include optics (notshown). These optics may include lenses, mirrors and/or other opticalhardware components to direct and/or confine the radiation emitted fromthe plurality of individual subsources (e.g. LEDs) to a specific area onprinting plate 830. This configuration may produce a stronger contrastbetween the dark and illuminated areas on printing plate 830, therebyincreasing accuracy of the exposure process.

In yet another exemplary embodiment 900, a stationary plate 930 may besubjected to irradiation from a single linear source 915 that isconfigured to pass over both the front side and back side of the plateat a speed that provides the desired time delay, with a controller 940that, for example, may turn the source on and off at the appropriatetimes or modulate the amount of radiation between a main exposureintensity and a back side exposure intensity, as needed. The plate maybe disposed on a substrate 960 in a horizontal system, as depicted inFIG. 9, or the system may be oriented vertically, as described in otherembodiments. It should be understood that the source may travel ineither direction, so long as the controller first commences irradiationat the leading edge of the back side. The structure for moving thesource may comprise, for example, a holder for the source mounted to abelt or chain that moves in a desired path. The source may move at adifferent speed (e.g. slower) when aligned over one side of the plate tocause an exposure than it does when it is traveling between the trailingedge of one side and the leading edge of the other side. In mostembodiments, because the time delay is generally a fraction of theoverall exposure time needed to expose the plate, this embodiment may becommercially practical only in processes in which the total exposure isspread over multiple passes.

It should be understood that the invention is not limited to anyparticular physical embodiment, and that the method of the invention ofincorporating an optimized delay between back side and front sideexposure may be performed in any system having any physicalconfiguration.

FIG. 10 illustrates an exemplary method for preparing a printing platein accordance with the invention, including in step 1000, commencingirradiating the back side of the printing plate, implementing a definedtime delay in step 1100, and then, immediately at the end of the timedelay, commencing irradiating the front side of the printing plate instep 1200. In one embodiment, the exposure may be carried out using amultitude of consecutive exposure steps, in which each step contributesa fraction of the total energy dose required for complete curing of theplate, as is known in the art. In accordance with the invention,however, each exposure step includes the requisite time delay. Thus, asdepicted in FIG. 10, in such an embodiment, each radiation step 1000 and1200 may comprise only a fraction of the total radiation desired, andsteps 1000, 1100, and 1200 may be repeated until the plate has beenexposed to the total amount of radiation desired.

In a method for optimizing the time delay for a specific type ofprinting plate at a specific set of exposure conditions, alsoillustrated in FIG. 10, the method further comprises creating a firstsample in step 1300, performing steps 1000, 1100, and 1200 on the sampleat the specific set of exposure conditions, creating a printcorresponding to the sample in step 1400, changing the time delay instep 1500, and then creating a new sample in step 1300 and performingsteps 1000, 1100, and 1200 on the new sample. Steps, 1300, 1000, 1100,1200, 1400, and 1500 may be repeated in sequence for a plurality ofsamples as many times as desired. Then, in step 1600, the optimum timedelay is selected. In some embodiments, the time delay corresponding tothe print having the smallest print dots may be optimal. In others, theoptimum time delay may correspond to a minimum value for the smallestprinted minimum dot diameter that coincides with a maximum value for thedot ground diameter for a range of time delay values.

Notably, as illustrated in FIG. 1, front side radiation source 110 andback side radiation source 120 do not spatially overlap one another.Thus, in relative motion systems, in addition to distance D between theleading edge 122 of light source 120 and the leading edge 122 of lightsource 110 (which may be an adjustable distance), there is alsopreferably a distance (d) between the trailing edge 124 of light source120 and leading edge 112 of light source 110. In other words, asillustrated in FIG. 13, at no time is any specific cross-sectionalcoordinate A, B, or C on the plate being exposed from both the frontside and the back side simultaneously, and thus the apparatus as a wholeis configured to prevent simultaneous irradiation of any specific crosssectional coordinate on the plate. As shown in FIG. 13, showing asnapshot of a particular portion of the plate during a specific momentin time during exposure, section A of plate 1330, which has ablated mask1320 on a top layer thereof, is irradiated by top source 1310, section Bis not irradiated by either source, and section C is irradiated bybottom source 1312, but there is no cross sectional coordinatecorresponding to a line parallel to A, B, or C, that is beingsimultaneously irradiated by both sources. However, because the timedelay is a fraction of the overall exposure time for the plate, bothsources are actively providing radiation to some portion of the platesimultaneously over at least a portion of the exposure time in mostsystems for most plate sizes. By making distance D shown in FIG. 1adjustable, the relative motion velocity between the plate and thesources can be varied within a certain range, without changing the timedelay between back and front exposure, because within that range D canbe adjusted to compensate for the change in relative velocity.

Such a configuration may be provided by a spatial configuration asdepicted with respect to FIG. 1, by a configuration of the controller,or by a combination thereof. Thus, in a system that does not create thetime delay using a spatial distance between the main and back sideradiation sources in combination with relative movement, but rather bypulsing stationary sources relative to a stationary plate, such as isdepicted in system 800, back side radiation source 820 (or one or moresubsources) may spatially overlap with front side radiation source 810(or one or more of subsources), but the controller is configured so thatsuch overlapping sources never actively irradiate the plate at the sametime.

Finally, while the time delay may be the same for each area of theplate, it should be understood that depending upon the configuration ofthe radiation sources, controller, and control scheme, one portion ofthe plate may be irradiated differently than another, if desired.

Processes for Increasing Back-Exposure

As described above, photo polymer printing plates typically need UVexposure from the front side and the back side. The front side exposureis applied through the mask, which holds the image information thatshall be imposed to the printing plate. The rear side is exposed to UVradiation through the rear side plastic substrate without any vignettingin order to build a polymerized support layer over the entire plate forthe fine printing details located on the plates front side. Thispolymerized support layer is called “floor.” The floor thicknessdetermines the relief depth for a given plate thickness.

Recent advances in technology for curing of photopolymer printing plateswith UV light has produced a variety of LED-based exposure units fromvarious suppliers. These units comprising LED UV light sources areincreasingly replacing so-called “bank” exposure devices thatincorporate fluorescent tube technology, in which light from thefluorescent tubes typically covers the complete plate surface at onetime. Because LEDs are more expensive and require more complex driverelectronics, while delivering a much higher UVpower-per-unit-surface-area than fluorescent tubes, many embodimentscomprise a light source that covers only one dimension of the polymerplate completely (e.g. width), while using relative motion between theplate and UV head to cover the second dimension (e.g. length), to ensureall surface areas of the plate are exposed to the UV light. One exampleof a state of the art exposure system is the XPS 5080 systemmanufactured by ESKO. This exposure system is equipped with 3 identicalUV heads: two for front side exposure, and one for rear side exposure.

Thus, in one embodiment of the invention, the exposure system comprisesthree (typically identical) UV heads (e.g. linear sources) comprising aplurality of LED sources: two heads directed to the front side (alsoreferred to herein as the “main exposure heads”), and one head directedto the rear side (also referred to herein as the “back exposure heads”),as shown in FIG. 1B. Each UV head covers only one dimension of the plate(e.g. width) completely, with reliance on relative motion between thesource and the plate to cover the other plate dimension (e.g. length),as described in other embodiments shown and described herein (see, e.g.,general configuration of the linear sources in FIG. 11).

Using the apparatus and method described above may present a challengewith respect to curing photopolymer printing plates having back sidesthat are less sensitive to UV radiation. Such lesser sensitivity mayarise from relatively higher photo-blocker content in the platerear-side plastic substrate. One way of overcoming this challenge is toapply back exposure energy to the plate rear side prior to the combinedback-main exposure steps.

For common standard polymer plates, the ratio (Rbf) between thepredetermined amount of energy applied to the rear (back) side andenergy applied to the front side of the plate to achieve desiredresults, ranges according to plate supplier information from 1:5 to1:40, respectively 20% to 2.5%, depending on the plate type and therelief depth to be achieved.

A standard polymer printing plate, such as for example, the DuPont®model DPR 045 plate, may require approximately 12 minutes of frontexposure and about 60 Seconds of back exposure for complete curing on abank exposure unit. In an LED UV exposure unit configured such as thosedescribed herein, and in particular in the XPS 5080 system manufacturedby Esko-Imaging Graphics GmbH, Rbf may be as high as 16%.

In certain highly-UV-sensitive plates, such as EXS model plates fromDuPont or FTF plates from Flint, a back exposure of only about 6 secondswould optimally be required using such systems, if the plates were nototherwise adjusted. As most bank exposure units control exposure time in1-second increments, adjusting the floor thickness with back exposurefor such high sensitive plates would be very difficult to achieve,without making some adjustments to the plate structure. Hence, platesuppliers now add a higher content of UV blocker into the rear sideUV-transmissive, dimensionally-stable plastic substrate of the plate,which blocks a substantial amount of UV from participating in the curingprocess from the rear side. Such adjustments by plate suppliers havepushed Rbf closer to, for example, approximately 50%.

Unfortunately, if the content of UV blocker inside the rear side plasticsubstrate is not controlled very precisely by the plate suppliers andvaries from one plate charge to another, it may be necessary to controlthe resulting floor thickness of the plates periodically by readjustingthe back exposure UV-energy accordingly.

Moreover, in many configurations, the plate typically lies flat on aglass table with its rear side adjacent the glass plate and isback-exposed through this glass plate. The glass plate typically has aUV transmission of around 80%, consuming 20% UV, thereby pushing the Rbfas high as, for example, approximately 62.5% for the exemplary platesdescribed above.

Another demand for increase of back exposure power arises from the factthat the exposure may be applied to the plate in several fractions.Using several smaller exposure-energy factions, the curing becomes lessefficient, making the total exposure energy required for complete curinghigher in comparison to an exposure applied in an uninterrupted step.The loss of efficiency due to using multiple exposure steps results inanother increase in Rbf, bringing it close to, for example,approximately 75% for the exemplary plates described above.

An embodiment equipped with three identical heads—one UV head for backexposure and two UV heads for main exposure—as depicted in FIG. 1B, hasan inherent Rbf of 50%, while the back exposure requirements for ahighly sensitive plate having the combined characteristics noted above,may require an Rbf of 75%.

It is thus not possible to adjust the floor thickness exclusivelythrough irradiance of the rear side UV Head using three identical headsoperating at the same power output, and for reasons described hereinlater, it may be desirable to use three identical heads with the samenominal power operating at that nominal power, for greatest efficiencyand productivity. One method for supplying the missing 25% of backexposure UV power is to provide a back-only exposure step to provide theadditional energy in an uninterrupted exposure into the rear side of theplate before the combined main and back exposure steps are commenced.

As shown in FIG. 1B, three identical UV heads are disposed relative tothe photopolymer plate to be cured. The plate comprises a mask 2132, aphoto sensitive polymer 2134 and a plastic substrate 2136 placed flat ona glass plate 2160, with the non-printing side of the plate in contactwith the surface of glass plate. Two heads for the main exposure 2110,which cure the front side of the polymer printing plate, are locatedabove the glass plate and the polymer plate thereon, and the third UVhead for back exposure 2120 is located under the glass plate 2160. Allparameters, like speed of the UV sources, distance between UV sources,the resulting time delay between the UV Source, the irradiance of theSources as well as the number of exposure cycles is controlled by thecontrol system 2140.

A standard exposure process comprises at least two exposure steps,wherein in each step the exposure heads move from a start position alongthe polymer plate, exposing the plate to actinic UV radiation withconstant speed V in the direction of the arrow shown in FIG. 1B, andafterwards returning in the direction opposite the arrow to their startposition without emitting radiation in the return pass. During thisprocess, in accordance with aspects of the invention described herein,the back exposure is applied to the polymer plate before the mainexposure by a precisely determined time difference. Accordingly, theback exposure head moves a constant distance in advance of the mainexposure head during polymer plate exposure, causing a constant delaytime between rear and front exposure. The distance between main UV headsand back exposure head, and consequently the delay time, is typicallyadjustable.

As described herein, total exposure time is determined by UV outputaperture of the UV heads in traveling direction, the speed by which theheads travel along the polymer plate and the number of passes the headmoved along the plate. The width of the UV output aperture divided bythe traveling speed results in the time that a pixel in the plate “sees”UV light. This time is called “pixel time” in the following text.

The intent of the foregoing method is to obtain a cure of the polymerplate that is superior to a simple front and backside exposure eachapplied in only one uninterrupted step. “Superior” in this case meansthe plate holds smaller printing details, which are fixed to the platefloor with higher stability and that do not bend during printing.Producing smaller print dots enables production of lighter highlights inthe print.

The back exposure energy is increased or decreased to adjust the floorthickness of the plate. This can be done either by adjusting theirradiance of the back exposure UV head, by adjusting the pixel time, orby adjusting both pixel time and irradiance. Due to the nature of thecuring process, changing the pixel time is disfavored, as it can alsoaffect the front exposure results, which may lead to unwanted curing andprinting results. Thus the preferred method for adjusting the platefloor thickness is to change the irradiance.

Productivity is a highly desirable characteristic of UV exposuredevices, and thus it is favorable to run the UV light sources at thenominal UV output in order to supply the energy to the plate requiredfor curing as fast as possible. As a consequence, both front exposure UVheads in the system depicted in FIG. 1B are preferably normally operatedat nominal UV power. Depending on the energy dose ratio Rbf between backand front side exposure, it is not always possible to reach the maximumproductivity, such as for example when the required back exposure energyis 75% of the energy required for front exposure, and the one head onthe back side can only deliver 50% of the front exposure energy. On theother hand, equipping the back side exposure with more UV heads isdisfavored, as this increases system manufacturing costs, because headscan only be added in integer numbers, which may cause too much rear sideUV power for most plates.

In the example discussed above, where the irradiance of the backexposure head cannot be increased any further, absent some other remedy,the UV output of the back exposure would have to be set to maximum andthe pixel time increased by 50% to reach the required floor thickness(with some corresponding change to the front exposure to adjust for theincreased time). This approach may affect the quality of the frontexposure curing result, and thus, it may be necessary to evaluate thecomplete set of exposing parameters to maintain desirable plate printingquality.

Alternatively the number of exposure cycles may be increased until theaccumulated rear side exposure energy is sufficient to cure the platefloor to the required thickness. But this approach will also increasethe front exposure energy, leading to higher energy input if the frontexposure irradiance stays the same as before the increase in the numberof cycles, which may lead to different curing and printing results. Alsoif the irradiance of the front exposure is reduced to maintain the frontexposure energy the same as before the increase of the number ofexposure cycles, this may lead to different curing and printing results,making this an imperfect solution.

Table 1 below provides a survey of different Rbf ratios for variousexemplary plate types. As shown in Table 1, the Rbf ratio increases forthicker plate materials. For thicker plate material and also for the newhighly-UV-sensitive materials, like the EXS plate from DuPont or the FTFplate from Flint, there is a need for further improving the efficiencyof the XPS exposure device. The plates listed in the table are merelyexamples, and the invention is not limited only to use with digitalflexographic printing plates listed.

TABLE 1 UV back side UV main side energy no of energy Ratio back sideIrradiance speed cycles main side Irradiance Back/Front Plate type[mJ/cm²] [mW/cm²] [mm/s] [n] [mJ/cm²] [mW/cm²] Rbf [%] DRAVE 045 3944136 9.9 4 9019 311 43.7 DRAVE 067 4350 150 9.9 4 9019 311 48.2

One preferred method for bringing more back side exposure energy intothe plate, without sacrificing the desired printing results of the platederived from the proper selection of exposure parameters, comprisesexecuting one or more additional back exposure steps prior to theconsecutive combined back and front exposure steps discussed herein.This additional back exposure step is preferably applied directly beforestarting the consecutive exposure steps.

Thus, for the processes described and depicted relative to FIG. 10, oneor more back-only exposure steps for adjusting the plate floor may besequentially performed immediately prior to step 1000. This methodeliminates any need for new time-consuming parameter evaluation of allparameters in the event of needing to adjust the floor. For example, inprocesses that include step 1300-1600 for characterizing a plurality ofsamples, one or more back-only exposure steps may be included to set thedesired amount of floor as part of the initial characterization of aparticular type of plate. Then, in the event a particular batch ofplates may require adjustment of the floor relative to the originalamount of back-only exposure characterized for that type of plate, theadjustment process may include only adjusting the back-only exposureamount, keeping all other parameters in all other steps the same. Whiledetermining the correct adjustment for the floor may include aniterative process of generating and evaluating one or more samples withvaried amounts of back-only exposure, adjustment of the floor may alsobe effected by merely increasing or decreasing the back-only exposureproportionally relative to the increase or decrease in floor desired.

The energy applied by the additional back exposure steps may becontrolled by the exposure time, namely by the speed by which the UVhead is moved along the plate rear side. In order to keep the timeperiod for this additional exposure step as short as possible, the UVirradiance of the rear exposure UV head is ideally operated at thenominal maximum.

The amount of additional back exposure UV-energy may be controlled bycontrolling the irradiance of the back exposure UV-heads, by adding moreback exposure steps (each step comprising a fractional amount of a totalthat is predetermined to be required to provide the desired floorthickness adjustment), or by some combination of irradiance, exposuretime (the speed by which the UV head is moved along the polymer platerear side), and the number of additional exposure steps.

Additional back exposure steps are added whenever the nominal backexposure irradiance is not sufficient for complete curing of the floorto the required thickness. The additional back exposure step or steps(as well as any and all of the method steps described herein) may beimplemented via the apparatus controller, which controller may be in theform of computer hardware programmed with software instructions forcausing the components of the apparatus to perform the subject steps.Although shown and described in connection with a certain embodiments ofthe invention described herein, it should be understood that exposureprocesses comprising additional back-exposure steps are not limited toany particular process or apparatus. Similarly, although the use ofadditional back-only exposure steps may be particularly suitable for UVexposure systems, this aspect of the invention is not limited to anyparticular radiation system.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

What is claimed:
 1. An apparatus for preparing a printing platecomprising a photosensitive polymer activated by exposure to actinicradiation, the printing plate having a non-printing back side and aprinting front side with a mask for defining an image to be printed, theprinting plate having a lateral width and a lateral length and disposedin a stationary position, the apparatus comprising: a plurality ofradiation sources configured to emit the actinic radiation toward theprinting plate in the stationary position, each of the radiation sourcescomprises a light emitting diode (LED) configured to emit actinicultraviolet (UV) radiation, the plurality of radiation sourcesincluding: a set of front LED sources positioned to emit radiationtoward the front side of the plate, the plurality of stationary frontLED sources together defining a collective irradiation field covering anarea at least coextensive with the lateral length and lateral width ofthe plate; a set of back LED sources positioned to emit radiation towardthe back side of the plate, the plurality of stationary back LED sourcestogether defining a collective irradiation field covering an area atleast coextensive with the lateral length and lateral width of theplate; and a controller connected to the plurality of radiation sourcesand configured to activate the plurality of radiation sources to emitthe actinic radiation in a predetermined pattern that includes a timedifference between activation of at least a portion of the set of backLED sources and activation of at least a portion of the set of front LEDsources.
 2. The apparatus of claim 1, further comprising a holder fordisposing the printing plate in the stationary position, the holdercomprising a material that is transmissive of the actinic radiation. 3.The apparatus of claim 2, wherein the holder defines a horizontalsurface relative to a ground on which the apparatus is disposed.
 4. Theapparatus of claim 2, wherein the holder defines a cylindrical surface.5. The apparatus of claim 1, further comprising a holder for disposingthe printing plate in the stationary position, wherein the holder isconfigured to hold the plate in a vertical orientation relative to aground on which the apparatus is disposed.
 6. The apparatus of claim 1,wherein the predetermined pattern includes all of the set of front LEDsources emitting actinic radiation simultaneously, all of the set ofback LED sources emitting actinic radiation simultaneously, or acombination thereof.
 7. The apparatus of claim 1, wherein each of theplurality of radiation sources are individually controllable orcontrollable in subsets smaller than an entirety of the collectiveirradiation field corresponding to each set.
 8. The apparatus of claim7, wherein the predetermined pattern comprises a sequence that mimicsrelative motion between the irradiation field and the plate.
 9. Theapparatus of claim 7, wherein the predetermined pattern precludessimultaneously irradiating the front and the back of the plate byrespective LEDs spatially aligned with one another relative to a samecross-sectional coordinate of the plate.
 10. The apparatus of claim 9,wherein the predetermined pattern includes activation of multipleportions of the set of front LED sources and the set of back LED sourcessimultaneously.
 11. The apparatus of claim 10, wherein the predeterminedpattern includes activation of the multiple portions simultaneously in apattern that mimics multiple carriages.
 12. The apparatus of claim 9,wherein the predetermined pattern includes activation of all of theplurality of back LED sources simultaneously and then all of theplurality of front LED sources simultaneously, with a predetermined timedelay applied between each front and back exposure.
 13. The apparatus ofclaim 1, wherein the predetermined pattern includes activating at leastsome of the plurality of radiation sources in a plurality of exposuresteps, each exposure step including emission of actinic radiation at anenergy less than a full amount of energy required to cure acorresponding side of the plate to a desired degree of curing.
 14. Theapparatus of claim 1, further comprising optics to direct and/or confinethe radiation emitted from each set of LED sources.
 15. The apparatus ofclaim 14, wherein the optics include mirrors.
 16. The apparatus of claim1, wherein the predetermined pattern includes delivery of a firstradiation intensity in one exposure step and a second, lesser exposureintensity in another exposure step.
 17. The apparatus of claim 16,wherein the apparatus is configured to deliver the first radiationintensity and the second, lesser exposure intensity with the same set ofLED sources.
 18. The apparatus of claim 16, wherein the set of front LEDsources has a first nominal radiation intensity and the set of back LEDsources has a second nominal radiation intensity.
 19. The apparatus ofclaim 18, wherein the predetermined pattern includes activation of theback LED point sources at the second nominal radiation intensity in oneexposure step, and activation of the back LED point sources at aradiation intensity less than the second nominal intensity in anotherexposure step.
 20. The apparatus of claim 1, wherein one or both of theset of front LED sources and the set of back LED sources are stationary.21. The apparatus of claim 1, wherein each set of LED sources comprisesa plurality of discrete arrays having a plurality of individual LEDpoint sources on each array, with the plurality of LED point sourcesarranged in a plurality of lines.
 22. The apparatus of claim 21, whereinthe plurality of LED point sources on each array are controllabletogether.
 23. The apparatus of claim 21, wherein the plurality of LEDpoint sources on each array are individually controllable.
 24. Theapparatus of claim 21, wherein the plurality of LED point sources oneach array are controllable in groups.
 25. The apparatus of claim 24,wherein each line of LED point sources in each array is separatelycontrollable.
 26. The apparatus of claim 24, wherein a radiationintensity generated by at least one line of LED point sources differsfrom a corresponding radiation intensity generated by at least one otherline of LED point sources for a same amount of input energy, wherein thecontroller is configured to control the radiation intensity produced byeach line to achieve an intended degree of homogeneity.
 27. A method ofcalibrating the apparatus of claim 26, the method comprisingperiodically characterizing radiation intensity generated by a pluralityof lines of LED point sources and adjusting input energy to therespective plurality of lines of LED point sources to account forvariations in the lines over time.
 28. The method of claim 27, whereinthe characterization is performed by positioning a sensor that measuresincident radiation at a predetermined distance from each line of LEDpoint sources.
 29. The method of claim 27, further comprising tailoringradiation intensity of one or more groups of the LED point sources tocompensate for variations in transmissivity of a structure that liesbetween the sources and the printing plate.
 30. A method for exposing aphotopolymer printing plate to actinic radiation from a UV LED radiationsource, the photopolymer printing plate having a non-printing back sideand a printing front side with a mask for defining an image to beprinted, the method comprising the steps of: a) positioning thephotopolymer printing plate in an exposure unit, wherein the exposureunit comprises a plurality of UV LED radiation sources; and b) exposingthe photopolymer printing plate through the mask to actinic radiationfrom the plurality of UV LED radiation sources to cure a portion of thephotopolymer in the plate in an exposure step during which the pluralityof UV LED radiation sources and the photocurable printing plate do notmove relative to each other, including providing a first exposureintensity in one exposure step and a second exposure intensity,different than the first exposure intensity, in another exposure step.31. The method of claim 30, wherein the exposure unit further comprisesoptics, including mirrors, wherein the exposure step comprises directingthe radiation emitted from the plurality of UV LED radiation sourceswith the mirrors.
 32. The method of claim 30, wherein the plurality ofUV LED radiation sources comprises one or more sets of LED sources, eachset of LED sources together defining a collective irradiation fieldcovering an area at least coextensive with a lateral length and lateralwidth of the plate.
 33. The method of claim 30, wherein the plurality ofUV LED radiation sources emits the actinic radiation at a firstintensity, wherein the actinic radiation reaching a photocurable surfaceof the printing plate is less than the first intensity.
 34. The methodof claim 33, wherein a portion of the actinic radiation emitted by theplurality of UV LED radiation sources is not transmitted by a surface onwhich the plate is mounted, blocked by a substrate on which thephotocurable polymer is disposed, or a combination thereof.
 35. Themethod of claim 30, comprising providing the first exposure intensity inone or more back exposure steps and the second exposure intensity in oneor more front exposure steps.
 36. The method of claim 30, comprisingperforming the first exposure step with the plurality of UV LEDradiation sources emitting at a nominal intensity, and the secondexposure step is performed with the plurality of UV LED radiationsources emitting at less than the nominal intensity.
 37. The method ofclaim 36, wherein the first exposure step and the second exposure stepare both back exposure steps.
 38. The method of claim 37, comprisingproviding a plurality of back exposure steps including at least a firstback-only exposure step and a second back exposure step followed by apredetermined time delay followed by a front exposure step, the methodcomprising adjusting the intensity of the one or more of the pluralityof UV LED radiation sources in at least one of the back exposure stepsto provide a predetermined ratio Rbf between front side and back sideexposure of the printing plate.
 39. A system for preparing a printingplate, the system comprising: an imager configured to define the imageon the mask; the apparatus of claim 3; a handling device configured tomove the plate from the imager to the holder.
 40. The system of claim39, wherein the set of back radiation sources are stationary.
 41. Thesystem of claim 39, wherein the set of front radiation sources areattached to a carriage configured to traverse the plate longitudinally.42. The system of claim 39, wherein the plurality of radiation sourcesare controllable in subsets smaller than an entirety of the collectiveirradiation field corresponding to each set.
 43. The system of claim 42,further comprising a sensor configured to measure incident radiation ata predetermined distance from each subset of LED sources.
 44. The systemof claim 43, wherein each subset of LED sources has a variable intensityconfigured to be varied by a factor relative to other subsets in theset, based upon the incident radiation measured by the sensor, to givethe radiation emitted by the set an intended degree of homogeneity.